HYDROUS CORDIERITE IN GRANULITES AND CRUSTAL MAGMA PRODUCTION

Huge volumes of S-type granitic and volcanic rocks, as well as many mi gmatites, are the products of fluid-absent partial melting in metasedi mentary protoliths at granulite-facies conditions. Thus, it is essenti al to know the controls on the fertility of metasedimentary rocks as m agma sources. In the present experiments on fluid-absent partial melti ng of biotite in metasedimentary rocks, cordierite proportion is inver sely correlated with melt fraction. At constant pressure (P) and tempe rature (T), this variation in melt proportion is a function of bulk-ro ck Mg number [100 Mg/(Mg + Fe)] and activity of alumina (a(Al2O3)). Ho wever, melt compositions are essentially unaffected. At 0.5 GPa, the c alculated H2O contents of cordierite decrease as T increases from 850 to 1000 degrees C. In aluminous metasedimentary rocks, the cordierite stability field considerably overlaps with the conditions of fluid-abs ent melting in the middle to lower crust, severely restricting melt pr oductivity. Consequently, metapelites may be much less fertile sources of granitic magma than metagraywackes.